“The flight crew diverted the flight to Kansas City out of an abundance of caution based upon suspicious behavior displayed by several passengers,” FBI spokesperson Bridget Patton in Kansas City said in a statement.

All passengers were taken off the plane and those in question were interviewed by law enforcement, Patton said. They were later re-booked to continue on a different flight, she said.

Asked what constituted suspicious behavior, Patton replied in an email that the passengers were “unruly.”

Southwest said in a statement that the crew decided to divert the flight “in response to several passengers who did not follow crew instruction upon takeoff and continued to exhibit suspicious behavior in flight.”

Security officials worldwide have been on high alert since Islamic State militants claimed responsibility for attacks last week in Paris that killed 129 people.

Russia has said the group was also responsible for the downing on Oct. 31 of a plane returning to St. Petersburg from the Sharm al-Sheikh resort in Egypt, killing all 224 on board.

After passengers left the plane, authorities swept the aircraft with a K-9 dog unit, Patton said. The plane, which landed in Kansas City at 7:50 a.m. CT (1350 GMT), was re-boarded and it departed at 9:20 a.m. CT for Los Angeles, she said.

It was not immediately clear how many passengers were on board the flight.